1. Field of the Invention
The invention relates to apparatus for the thermoplastic processing of thermoplastic polymeric resins and more particularly relates to such apparatus including an improved means of maintaining the thermoplasticity of the resin during processing.
2. Brief Description of the Prior Art
The prior art literature is replete with descriptions of apparatus for thermoplastically processing thermoplastic, synthetic, polymeric resins; see for example the descriptions given in the U.S. Pat. Nos. 2,859,476; 3,308,508; 3,404,432; 3,820,927; 3,402,427 and 4,512,730.
In general, such apparatus includes resin extruders, injection molding apparatus, blow-molding apparatus, thermoplastic resin coating devices, resin laminators and like apparatus. Processing in such apparatus may include mixing, plasticizing, shaping or simply carrying the resin in a thermoplastic state from one point to another point.
In common to all of the above-described apparatus and devices for processing thermoplastic polymeric resins are associated means for obtaining and maintaining thermoplasticity of the resin during processing, such as the shaping of the resin material into an article. Maintenance of thermoplasticity may require both heating and cooling of the resin at various times in the processing of the resin. Cooling may be needed to avoid thermal degradation of the resin. Almost all of the resin processing techniques rely at least in part on heating or cooling of the polymeric resin by thermal transfer through the agency of a metal surface, part of the processing apparatus. Heat is generally applied to an outside surface of the metal apparatus by concentrated heat sources such as band heaters, or from within the body of the metal part by heater rods or circulating, heated fluids such as air, water or other chemical liquids. In all cases, the metal heat transferring components have to be of substantial thickness and mass to resist extreme pressures and mechanical forces. The large metal mass responds slowly to changes in thermal input or cooling so that precise control of narrow temperature ranges is difficult to control. Also, when temperature differences are desired in adjacent zones of the same apparatus, it is difficult to localize the particular and different temperatures to be maintained for appreciable periods of time. This shortcoming is especially troublesome for relatively complex processing techniques and apparatus, such as in the injection molding of large parts or in the co-extrusion of multiple-layered film laminations of diverse polymeric resins where the resins employed in the different layers have substantially different temperature requirements for processing.
The improved apparatus of the invention includes as a heater means thin film heaters which may be mounted in close association with the thermoplastic polymeric resins being processed in the apparatus. Heavy metal components to achieve thermal transfer to the resin are not necessary. There can be a saving of weight, materials and labor in manufacture. With the closer juxtaposition of the heating element in the subject plastic, a closer control of resin temperature is maintainable with quicker response times to maintain a pre-determined resin temperature, even in adjacent but different zones or localities. The lower thermal mass of the heating elements is more responsive to cooling or changes from heating to cooling or cooling to heating.